DS1501/DS1511
071098 4/22
care is taken with the data bus to avoid bus contention.
If OE is low prior to a high to low transition on WE, the
data bus can become active with read data defined by
the address inputs. A low transition on WE will then dis-
able the outputs t
WEZ
(WE data off time) after WE goes
active. (See Table 1.)
DATA RETENTION MODE
The 5–volt device is fully accessible and data can be
written and read only when V
CC
is greater than V
PF
.
However, when V
CC
falls below the power fail point V
PF
(point at which write protection occurs) the internal clock
registers and SRAM are blocked from any access.
While in the data retention mode, all inputs are dont
cares and outputs go to a high–Z state. When V
CC
falls
below the greater of V
BAT
and V
BAUX
, device power is
switched from the V
CC
pin to either the V
BAT
or V
BAUX
pin. RTC operation and SRAM data are maintained
from the battery until V
CC
is returned to nominal levels.
The 3.3–volt device is fully accessible and data can be
written and read only when V
CC
is greater than V
PF
.
However, when V
CC
falls below V
PF
, access to the
device is inhibited. If V
PF
is less than V
BAT
and V
BAUX
,
the device power is switched from V
CC
to the greater of
V
BAT
and V
BAUX
when V
CC
drops below V
PF
. If V
PF
is
greater than V
BAT
and V
BAUX
, the device power is
switched from V
CC
to the larger of V
BAT
and V
BAUX
when V
CC
drops below the larger of V
BAT
and V
BAUX
.
RTC operation and SRAM data are maintained from the
battery until V
CC
is returned to nominal levels. (See
Table 1.)
All control, data, and address signals must be powered
down when V
CC
is absent.
AUXILIARY BATTERY
The V
BAUX
input is provided to supply power from an
auxiliary battery for the DS1501/DS1511 kickstart and
SQW output features in the absence of V
CC
. This power
source must be available in order to use these auxiliary
features when V
CC
is not applied to the device.
This auxiliary battery may be used as the primary
backup power source for maintaining the clock/calen-
dar and extended user RAM. This occurs if the V
BAT
pin
is at a lower voltage than V
BAUX
. If the DS1501/DS1511
is to be backed–up using a single battery with the auxil-
iary features enabled, then V
BAUX
should be used and
V
BAT
should be grounded. If V
BAUX
is not to be used, it
should be grounded.
POWER ON RESET
A temperature compensated comparator circuit moni-
tors the level of V
CC
. When V
CC
falls to the write protec-
tion voltage, the RST signal (open drain) is pulled low.
When V
CC
returns to nominal levels, the RST signal
continues to be pulled low for a period of 40 ms to
200 ms. The power on reset function is independent of
the RTC oscillator and thus is operational whether or not
the oscillator is enabled.